Osteoporosis is the most common metabolic disorder of bone. The World Health Organization (WHO) estimates that 70 million people worldwide have osteoporosis. The annual worldwide incidence of hip fracture is 1.5 million, a number projected to grow to 2.6 million by 2025 and to 4.5 million by 2050. Bone strength is mostly dependent on its mineral content and therefore the measurement of bone mineral content is important to predict osteoporosis-related fractures. The most frequent site involved in fractures is the thoraco-lumbar junction, with mid-thoracic spine being the second most common site. The spine is the most sensitive site and is usually the first site to experience osteoporosis fractures. The bone turnover rate of the spinal trabecular bone is significantly higher than the cortical bone (the spinal trabecular bone remodeling rate is 5-10 times higher than the cortical bone).
Quantitative computed tomography (QCT) is a technique for three-dimensional bone mineral density (BMD) measurement. The bone density can be measured separately in the trabecular and cortical bone compartments. Therefore, thoracic quantitative computed tomography (TQCT) is an important technique for estimating and monitoring bone strength, especially in thoracic BMD measurement.
Cardiovascular diseases in general and coronary artery disease in particular remain the number one cause of death and disability in all industrialized and in many developing countries all over the world. In 2002, coronary artery disease, also referred to as coronary heart disease (CHD), caused 7.2 million deaths worldwide. Each year there are about 5.8 million new CHD cases, and about 40 million individuals with preventable CHD are alive today. The presence of calcium deposits in the coronary arteries has been well recognized as a marker of atherosclerotic coronary artery disease and gives independent incremental information, in addition to the traditional risk factors, in predicting cardiac events. Electron beam computed tomography (EBCT) has successfully been used to quantify plaque burden (by scoring calcification in the coronary arteries) over the past 20 years. Currently the cardiovascular CT scan is used widely to estimate calcium score and these scanners have obtained significantly higher temporal, special and contrast resolution with a significant decrease in radiation dosage using prospective triggering techniques. Current calcium scoring systems have a step-wise increase in scores that leads to decreased reproducibility using the Agatston score method. The most common current scoring system is one that uses the peak CT density, with a score of 1 applied for HU of 130-199, 2 for 200-299, 3 for 300-399 and 4 for 400 and greater. This impairs rescan reproducibility.
There is a significant positive association between BMD values obtained from the thoracic and lumbar spinal vertebrae (r=0.93 in female and r=0.88 in male). Even the cervical spine BMD is also reported to be significantly correlated with the lumbar BMD (r=0.66 to 0.87). In this aspect, it has been shown that the TQCT with CAC scans give an adequate measure of BMD with a high degree of precision and reproducibility. Therefore, the thoracic cardiac scan is likely to be a useful ancillary assessment of bone mineral status in the population in general.
However, a significant difference in the BMD measurements found using the TQCT technique and the lumbar quantitative computed tomography (LQCT) technique has been reported (i.e., a different of 5.8-13.9%) and our study result (9.6% in female and 10.4% in male). This variation can be caused by various factors: such as scanner type, scan protocol, scan technique parameter, patient's age contribution, patient's status, etc. Bone density and loss rate have significant differences between lumbar and thoracic spines, therefore one cannot use current LQCT standard in TQCT measurement directly. The validating study is necessary to develop a method and reference standard by TQCT scan.